(VOA) -- United Nations officials and international aid workers fear the Turkish government is gearing up to shutter more Western nongovernment organizations working with Syrian refugees and overseeing cross-border relief missions inside Syria following the expulsion Tuesday of Mercy Corps, one of the world's biggest humanitarian nonprofits.

The offices of more than half-dozen major Western NGOs and aid organizations with offices in the southern Turkish border towns of Gaziantep and Hatay were visited this week by Turkish officials, who demanded registration documents as well as copies of staff lists. The officials also scrutinized the work permits of international employees.

At least two international NGOs have been ordered so far to undergo the laborious bureaucratic process of re-registering their presence in the country, according to several aid workers.

U.N. officials say they suspect all Western NGOs are likely to be instructed shortly to re-register and it is unclear whether they will be allowed in the meantime to continue working with more than 2 million Syrian refugees in Turkey or to transport relief supplies to hundreds of thousands of displaced civilians inside war-wracked Syria.

U.N. officials told VOA they are urgently seeking clarification from the Turkish authorities.

Internal 'read-out' leaked

According to an internal "read-out" by the U.N.'s Office for the Coordination of Humanitarian Affairs, or OHCA, Turkey's interior ministry "is planning on canceling all existing INGO registrations and asking INGOs to resubmit registration requests following new rules and regulations within three months."

The OHCA read-out, which was leaked to VOA, warns that the Turkish government is likely to use this process as a "way to choose which organizations they want to keep in country."

According to OHCA, the interior ministry recently held a meeting of all Turkey's regional governors to discuss new rules and regulations for INGOs -- the first time, U.N. officials noted, that the Turkish government has organized a governor-level coordination meeting about international relief organizations.

Some of the most respected organizations on the international aid scene were among the agencies inspected this week, prompting mounting fears among aid workers that they are being targeted. They include: the International Rescue Committee, CARE International, Counterpart and Global Communities and International Medical Corps -- all are U.S.-based. The British NGO Integrity also was inspected.

Several other NGOs have been inspected but asked not to be listed for fear of incurring Ankara's wrath.

"Our offices in Gaziantep were visited this week," confirmed Holly Frew, a spokeswoman for Care. "We have heard other international NGOs have been inspected as well," she told VOA.

Frew said Care has not been asked to leave, but she said it is unusual for so many relief organizations to be inspected at the same time.

The spokeswoman for another aid organization confirmed her group had been visited, too. "Our offices were visited earlier this week by local authorities," she said. "They were checking staff work permits."

She said the organization's staff in Turkey believe it was "a routine visit and one that the authorities have the right to perform. Our documents were in order, and we had no problems. We have traditionally had a good working relationship with Turkish authorities and at this moment there is no reason to believe that will change."

'This really is ominous'

According to another aid worker, inspections by Turkish authorities in the past have taken place on an annual basis. "Since the Mercy Corps expulsion there has been an escalation in INGO inspections," he said. "This really is ominous," he added.

Mercy Corps, which has been working in Turkey since 2012, was informed abruptly on Tuesday by the Turkish interior ministry that it no longer had permission to work in Turkey and was ordered to shut down its operations immediately. The aid group said Turkish authorities offered no reason for the sudden closure and noted it had been working in close cooperation with the Turkish government.

In recent months pro-government Turkish newspapers have rounded on Western NGOs, and amid a mounting press clamor alleged Mercy Corps and other international aid groups have been helping terrorists and conspiring against the government of President Recep Tayyip Erdogan.

Mercy Corps, which is based in Portland, Oregon, and other relief organizations have vehemently denied the claims.

"We have every confidence in the impartiality and the integrity of our operations. We're not a political organization and our reason for being is to deliver assistance to civilians who need it the most," Christine Bragale, Mercy Corps' director of media relations, told Britain's Daily Telegraph earlier this week.

Mercy Corps, which has received funding for its Syria relief operations from the U.S. and European governments, spent $34 million last year on Syria-related missions. Mercy Corps has had to lay off 300 relief workers because of the expulsion.

U.N. officials say the anti-NGO press campaign, which they suspect is orchestrated, has grown in ferocity and has spread to local newspapers across the country.

In the OHCA report it is noted: "Negative articles about INGOs (similar to those printed in Hatay) are now being printed in Gaziantep." Both Hatay and Gaziantep are relief hubs for Syria, and international NGOs organizing cross-border efforts generally base out of either town.

A Turkish government official told Reuters that the decision to expel Mercy Corps was "technical," arguing the NGO failed to meet documentation requirements. But aid workers and U.N. officials fear there was nothing technical about the expulsion of Mercy Corps.

US concerns about expulsion

State Department spokesman Mark Toner midweek said U.S. concerns about the expulsion of Mercy Corps have been communicated to Ankara as well as anxieties about the "impact it will have on critical humanitarian assistance to vulnerable populations."

Relations between international NGOs and Erdogan's government have been fraught and often tense since the Syrian conflict started six years ago, aid workers say.

"The government has never wanted us in the country," said an international aid worker, who asked not to be identified. "It is horrible when aid becomes a pawn in a political game."

The registration process has often been difficult and Ankara has been strict about regulations governing what NGOs can do, where they can go and where they can send aid. The United Nations refugee agency has been blocked frequently from aiding Syrian refugees who are living outside official camps, U.N. officials say.

But relations have become much more fraught in recent months. Ankara has blocked international aid agencies from working in a Turkish-controlled zone carved out in northern Syria since August, preferring to work with Turkish NGOs favored by Erdogan, many of which are Islamist in ideology.

Analysts have noted that the press campaign against international relief organizations and Western NGOs tends to become more vehement when the Turkish government comes under criticism for alleged human rights abuses by Western governments.

Turkish nonprofits shuttered

The Erdogan government has targeted also domestic NGOs, shuttering hundreds of Turkish nonprofits since last July's failed coup attempt, claiming they are linked to the coup plotters, including exiled cleric Muhammed Fethullah Gulen, who lives in the U.S. and is accused by Erdogan of masterminding the military coup bid.

While acknowledging the latest moves against international NGOs may be part of what critics argue is a general crackdown on civil society in Turkey, Bassam al-Kuwaitli, a former member of the opposition Syrian National Council, suspects Ankara is determined to control international relief missions inside northern Syria.

"I think a lot of this has to do with Western relief assistance going to areas inside Syria controlled by the Kurdish-led Syrian Democratic Forces," he said. He notes pro-government Turkish newspapers this week accused Mercy Corps of assisting Turkey's banned Kurdistan Workers' Party, or PKK. The Erdogan government views U.S.-backed Syrian Kurdish militias fighting in Syria just as PKK offshoots.

An international aid worker noted the increased friction coincides with the arrival this week of a contingent of U.S. Marines, pre-positioning for an SDF assault on Raqqa.

Ankara has demanded that a Turkish-led Syrian Arab force mount the assault on Raqqa, the de facto capital of the Islamic State terror group, rather than Kurdish-led militias.